<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <link href="E:/idea_workspace/study/Bootstrap/BootstrapDemo/src/main/resources/static/bootstrap-3.3.7-dist/css/bootstrap.min.css"
          rel="stylesheet">
</head>
<body>
<form action="#">
    <select id="dropdown" class="form-control" multiple>
        <option>pick one</option>
        <option>one</option>
        <option>two</option>
        <option>three</option>
<!--        <optgroup label=""-->
    </select>
    <span id="helpfield" class="help-block">this text describes a form field</span>
    <input type="text" id="inputwithhelpblock" class="form-control" aria-describedby="helpfield">
</form>

<script src="E:/idea_workspace/study/Bootstrap/BootstrapDemo/src/main/resources/static/bootstrap-3.3.7-dist/js/bootstrap.min.js"/>
</body>
</html>